Under existing medical practice, each time an implantable cardioverter defibrillator (ICD) is implanted in a human patient, an intraoperative testing procedure is attempted in order to determine a minimum defibrillation threshold (DFT) in terms of the number of joules of electrical energy required to successfully defibrillate a patient for the particular electrode lead combination which has been implanted in that patient. The intraoperative testing procedure involves inducing ventricular fibrillation in the heart and then immediately delivering a defibrillation countershock through the implanted electrode leads of a specified initial threshold energy, for example, 20 joules for a monophasic countershock. If defibrillation is successful, a recovery period is provided for the patient and the procedure is usually repeated a small number of times using successively lower threshold energies until the defibrillation countershock is not successful or the threshold energy is lower than about 10 joules. If defibrillation is not successful subsequent countershocks of 35 joules or more are immediately delivered to resuscitate the patient. After a recovery period, the procedure is repeated using a higher initial threshold energy, for example, 25 joules. It is also possible that during the recovery period prior to attempting a higher initial threshold energy, the electrophysiologist may attempt to lower the DFT for that patient by moving or changing the electrode leads.
The intraoperative testing procedure is designed to accomplish a number of objectives, including patient screening and establishing a minimum DFT for that patient. Typically, if more than 30 to 35 Joules are required for successful defibrillation with a monophasic countershock the patient is not considered to be a good candidate for an ICD and alternative treatments are used. Otherwise the lowest energy countershock that results in successful defibrillation is considered to be the DFT for that patient. The use of the lowest energy possible for a defibrillation countershock is premised on the accepted guideline that a countershock which can defibrillate at a lower energy decreases the likelihood of damaged to the myocardial tissue of the heart.
Recent efforts to improve the efficiency of ICD's have led manufacturer's to produce ICD's which are small enough to be implanted in the pectoral region, thereby enabling the housing of the ICD to form a subcutaneous electrode, such as described in U.S. Pat. No. 5,405,363. Further developments in the industry have led to active housing electrode emulators as disclosed in U.S. Pat. No. 5,411,539 to Neisz in order to simulate an active housing electrode during the testing procedure. The system described by Neisz in the '539 patent provides a largely reusable active housing electrode emulator for screening patients for suitability for permanent implantation with an ICD having an active housing electrode. The system disclosed in the '539 patent has a reusable, sterilizable conductive can conforming to the dimensions of the ICD desired to be implanted. The reusable emulator has an electrical and mechanical attachment mechanism to connect to a standard ICD lead. Once DFT testing is completed, the ICD lead is then disposed of.
As described in the Neisz '539 patent, the rational for using reusable emulation housing electrodes is that they will save money by not having to dispose of the emulation housing electrodes after the testing of a patient is completed. In actuality, the use of reusable housing electrodes may cost more in the long run because such emulation housing electrodes must be constructed of high quality, expensive materials, for example platinum or MP35N. Over time, the housing electrode will begin to anodize which causes the impedance to change, which in turn changes the electrical characteristics of the electrode. This anodization may have a profound effect on the test results and therefore, the emulator must be disposed of when anodization occurs.
Additionally, the Neisz '539 patent teaches of using a standard ICD lead and then disposing of such lead when DFT testing is complete. Specifically, column 7, lines 50-51, state that "After testing is complete, the lead component 90 may be disposed of . . . ". Such leads are very expensive because they are designed to be implanted into patient's body. The leads must be made of governmentally approved materials that are biocompatible and durable enough to last in contact with body fluids for many years. Also, the leads contain a header that includes a pair of female receptacles and expensive through hole connections for connecting the female receptacles of the header to conductive wires in the lead.
While the existing techniques for performing intraoperative testing using an active housing electrode emulator to establish a minimum DFT for a patient are acceptable, it would be advantageous to provide an active housing electrode emulator which improves performance at a reduced per use cost.